This invention relates to a wheel service machine and a method for locking a wheel to a wheel-holder unit. This invention also provides a device for locking the wheel to a wheel-holder unit.
The invention applies to the field of equipment for tyre repair specialists and in particular to that of the wheel service machines. It should be noted that the term wheel (for vehicle) means the coupling between a tyre and a corresponding rim (that is, the overall tyre/rim).
Amongst the prior art wheel service machines there are two main types:                balancing machines, designed to measure the static and/or dynamic unbalancing of a wheel;        machines for fitting and removing a tyre on/from a corresponding rim (also known in the prior art as a “tyre changer machine”).        
The prior art wheel service machines usually comprise a wheel-holder unit having a supporting shaft, designed for rotating a wheel (or a rim) about a main axis of rotation, and a locking device, designed for fixing the wheel (or the rim) to the supporting shaft for rotating together with it, avoiding relative movements between wheel and wheel-holder unit which might cause malfunctions of the wheel service machine and/or damage to the wheel.
Wheel service machines are known in the prior art comprising a manual locking device, such as, for example, shown in patent document U.S. Pat. No. 4,768,909, which illustrates a locking device for a balancing machine. The locking device comprises a pair of threaded elements, movable radially towards (or away from) a supporting shaft, which is also threaded, for coupling with (or uncoupling from) it. When the threaded elements are not coupled to the supporting shaft, the locking device is free to slide along the supporting shaft. When the threaded elements are coupled the supporting shaft, the locking device is movable by roto-translation along the supporting shaft, to move into contact with a portion of the wheel (or the rim) and lock the position against a contact element (for example, a centring cone or a supporting flange).
These manual locking devices have considerable drawbacks. A first drawback is linked to the time necessary for the locking. A further drawback is linked to the forces which a user must apply to activate the device. Lastly, these manual devices suffer from a safety problem, since the resistance of the system is linked to the force with which the locking element is clamped.
In order to deal with these drawbacks, wheel service machines are known in the prior art comprising a motor-driven locking device, typically activated by an actuator. By way of example, the patent document EP2218591B1, in the name of the same Applicant, illustrates a locking device (for a tyre changer machine) comprising an actuator. The actuator is designed to move the locking device along a supporting shaft and bringing it into contact with a wheel (or a rim) for fixing it to a supporting flange preventing the relative rotation.
These locking devices have some drawbacks. A first drawback is linked to the increase in weight of the locking device, which limits the ease of transport and use. A further drawback is linked to the reliability of these devices, which are normally powered by a battery, which may run flat or become faulty. These devices are also unsuitable for use on balancing machines because they are per se significantly unbalanced.
There are also prior art wheel service machines wherein an actuator positioned on the machine is designed to move the locking device. Typically, the actuator is powered through the electricity network. The supporting shaft has a plurality of fastening elements, movable in a longitudinal direction defined by the supporting shaft and designed for coupling with a locking element and moving it between an active position, wherein it is in contact with a wheel (or with a rim) for fixing it to a supporting flange and a configuration of non-interference with the wheel (or with the rim). Typically, the actuator which moves the fastening elements is actuated by a control pedal located at the side of the machine. Typically, to lock the wheel to the supporting shaft of a balancing machine it is necessary keep the wheel raised, positioning with a hand the locking element and simultaneously pressing the control pedal. This solution is obviously inconvenient. The view of the pedal, both in tyre changer machines and in balancing machines, may be obstructed by the presence of the wheel; the user is thus required to operate in conditions of poor visibility, and the locking operation may be particularly difficult if working with heavy and/or large wheels. This drawback is particularly serious if the wheel service machine is a balancing machine with a horizontal axis (that is, a balancing machine wherein the supporting shaft is oriented perpendicularly to the weight force).